Typically, liquid ink includes pigment-based colorants and/or dye-based colorants dissolved or dispersed in a liquid vehicle. Dyes are freely soluble in the liquid vehicle whereas pigments need to be dispersed using dispersants. Dispersants may be small molecules, surfactants or polymeric resins. Dispersants provide stability to dispersions by keeping the pigment particles apart by electrostatic and/or steric repulsion. The dispersants may be covalently attached to the surface of the pigment particles or simply adsorbed by physical forces. Polymeric dispersants are increasingly used in commercial pigment dispersions.
Traditionally, pigment vendors have used the same polymer that works in dispersing one pigment for all pigments. This practice continues for various reasons, including cost effectiveness associated with using one polymer. However, that same polymer may not be able to effectively produce stable dispersions with all the different pigments. For example, inks made with one polymer to form stable cyan and black dispersions may produce less stable yellow dispersions and unstable magenta dispersions.
In addition, pigment vendors have tried increasing the amount of dispersant to increase the stability of the ink formulations. Stability does not always result. Instead, higher viscosity or nozzle health problems may result. Other attempts to improve stability have utilized covalent bonding of polymers to pigments. However, this approach adds to the cost and complexity of the dispersion process.
Accordingly, it would be advantageous to have a rational approach to polymer design for preparing stable ink dispersions.